The invention relates to a method for operating a variable displacement compressor and a system with a variable displacement compressor.
Variable displacement compressors are known in the art. Variable displacement compressors of the swash-plate type are widely used in automotive air conditioners. Such compressors include a displacement control mechanism, which operates to maintain the exit pressure of an evaporator or the suction pressure of a compressor associated with the exit pressure at a predetermined target value. The exit pressure of an evaporator or the suction pressure of a compressor associated with the exit pressure reflects the magnitude of the cooling load. The variable control mechanism controls the exit pressure of the evaporator or the suction pressure to adjust the inclination angle of the swash plate of the compressor which adjusts the displacement of the compressor. A compressor used in vehicles is usually driven by the vehicle engine. Being a main consumer of engine power, the load of the compressor to the engine has to be reduced under exceptional conditions, e.g. during acceleration or moving uphill or when starting the engine.
U.S. Pat. No. 4,578,960 B2 teaches a displacement control method for a variable displacement compressor which is incorporated in a refrigerant circuit of a vehicle air conditioning system. The displacement of the compressor is switched dependent on the temperature of the refrigerant. A control valve is provided for changes of the displacement of the compressor by changing the valve opening degree. The displacement is switched to a value between 30-50% of the maximum displacement if the temperature of the refrigerant drops below −1° C. If the temperature of the refrigerant is above 3° C., the compressor is operated at its maximum displacement.
The U.S. Pat. No. 6,484,520 B2 discloses a displacement control apparatus, wherein a variable displacement compressor of a swash plate type is employed. A drive shaft of the compressor is driven by a vehicle engine. A control valve changes the displacement of the compressor. An air conditioner controller produces a torque setting signal to the compressor controller which changes the valve opening and subsequently the displacement of the compressor. Thus, the compressor is controlled according to the torque.
It is desirable to provide a reliable and fast method for operating a variable displacement compressor. It is also desirable to provide a reliable system for operating a compressor. It is also desirable to provide a suitable computer program as well as a computer program product.
According to a first aspect of the invention a method for operating a variable-displacement compressor is provided, wherein a volume of a compression chamber of a cylinder is varied during a compressor cycle, also called compression cycle, wherein during the cycle said cylinder is alternately coupled to a low pressure side and a high pressure side of said compressor. The method comprises the steps of estimating a first thermodynamical state of said low pressure side and a second thermodynamical state of said high pressure side, estimating a current compression ratio of said compressor, calculating said displacement as a function of said compression ratio, estimating a compressor torque depending on the calculated displacement, and providing said compressor torque to a control unit for operating the compressor or for operating an engine coupled to the compressor. The control unit can operate the compressor directly or indirectly. For instance, the control unit can be an engine control unit where the engine drives the compressor. Knowing the displacement is useful when estimating the compressor torque, particularly when the compressor is employed in an air conditioning system of a vehicle. If the compressor torque is known, the control unit, such as an electronic control unit (ECU) or an engine control unit of a vehicle or the like, can easily compensate for the torque, thus making the control of the respective component, for instance the engine, easier. The drivability of the vehicle can also be improved. Particularly, the drivability is improved in the way that jerk problems are decreased. Other benefits gained by better compressor torque estimation which can be sent to an engine management system of a vehicle are decreased emissions of the engine as well as a decreased fuel consumption of the vehicle.
Additionally, the preferred method can easily be transferred to other systems. When employed into an air conditioning system, the method allows for better controlling of an evaporator coupled to the air conditioning system, particularly for better controlling of the evaporator temperature. Control of the evaporator can mean e.g. control of a fluid temperature, e.g. air temperature directly downstream the evaporator between the evaporator and the compressor and/or control of a coolant pressure between the evaporator and the compressor, which pressure can preferably correlate with the air temperature. Preferably, the evaporator can also be controlled by a control unit. For instance, the compressor control and torque estimation can be placed in the same control unit as the engine management software, or, for instance, a climate control unit can control the evaporator. A fluid which is compressed in the compressor can be a gas or a liquid.
Other than non-physical models, which depend strongly on measured data, the proposed method is based on a physical model of the compressor. As a physical model, the method is rather independent of the ambient environment of the compressor. A transfer of the method to another system can be done easily and fast, which is time and cost efficient. Knowing the actual displacement of the compressor is not only favorable for estimating the compressor torque but also for easier controlling an engine of an engine driven compressor, for instance when the compressor is coupled to a vehicular air conditioning system. Problems with a wind-up of regulator integrated parts of an evaporator, which can arise if the displacement is not known, can be avoided. Such problems can occur if the compressor is already at its maximum displacement and the requested cooling power is higher than can be provided by the compressor.
Preferably, the compressor is a swash-plate type compressor. However, other variable displacement compressor types are possible.
Only few, preferably three, signals are needed for the modeling on which the preferred method is based. The additional parameters for the model can be easily determined by thermodynamic property tables of the fluid being compressed in the compressor, such as a common refrigerant 1,1,1,2-tetrafluorethane, also known under the trade name R-134a, for example. This is reasonable, if the fluid cannot thermodynamically be treated as an ideal gas. If a fluid is used which can thermodynamically be treated as ideal gas, the parameters can be computed by thermodynamic formulas containing constant thermal capacities as known in the art and which can be found in thermodynamic textbooks.
The state on the low pressure side of the compressor can be determined with the help of an ambient temperature, particular an air temperature, downstream after an evaporator connected to the compressor inlet. The state on the high pressure side can be determined with the help of a measured pressure and the assumption that the compressor work is isentropic with a very high efficiency
Additionally, to estimate the torque, the engine speed has to be known. It is not necessary to know the mass flow or the actual swash-plate angle, which parameters are difficult to determine in praxis. Favorably, the method can be used quite general, which means that the estimation of the compressor torque does not depend on the ambient in which the compressor is utilized. For example, it can be utilized in various vehicles or different engines. Further, there is no influence on the model in different climes, so the model can be applied to compressors which are used in an air conditioning system in different countries all around the world. If a compressor of similar type is used, there is no need to build a new model. Developing a new model is very time consuming. Therefore, a general model such as used in the preferred method is cost-efficient and time-saving.
Only a few input signals are necessary. Thus, the model is easy to analyze. Furthermore, only a few sensors are necessary which allow for an economic air conditioning system, particularly in high-volume series which are typical in vehicle production.
Favorably, said first thermodynamical state on the low pressure side is extracted at least from a temperature, characteristic of a fluid temperature at an inlet of the compressor. Preferably, the temperature is an air temperature measured at an evaporator being in fluid connection with said inlet. From said temperature an input pressure value of said fluid can be determined. It might be possible to combine one of the temperature and pressure with another appropriate measure, for instance air flow or the like, to get an improved estimate.
A specific enthalpy, a specific volume, a specific internal energy and a specific entropy of said fluid at said inlet can be determined, depending on said temperature value and said pressure value. These parameters can be extracted from thermodynamic property tables for the fluid, e.g. the refrigerant being compressed in the compressor.
Favorably, said second thermodynamical state on the high pressure side is extracted from a pressure value of the fluid at an outlet on the high pressure side of said compressor.
Preferably, the pressure value is measured at the outlet of the compressor. An entropy at said outlet is set equal or at least approximately equal to an entropy at the inlet. An enthalpy can be determined depending on said pressure value and said entropy. An isentropic efficiency can also be calculated.
An outlet temperature value of said fluid at said outlet is determined depending on said enthalpy and said pressure value.
Finally, a specific enthalpy, a specific volume and a specific internal energy of said fluid at said outlet can be determined, depending on said temperature value and said pressure value. The parameters can be drawn from thermodynamic property tables describing the thermodynamic properties of the fluid compressed by the compressor.
Said displacement of the compressor can easily be determined as a product of a volumetric efficiency and said compression ratio and a constant characterizing a constant volume in the size of the minimum displacement.
Said compressor torque can be extracted from an energy balance on the compressor.
The torque can be extracted from said calculated displacement, said specific enthalpy at said inlet, said specific enthalpy at said outlet, said specific energy at said inlet and said specific energy at said outlet. The torque can be compensated for operating said compressor efficiently, particularly in a vehicle.
According to another aspect of the invention, a system comprising a variable displacement compressor is provided, wherein an evaporator is in fluid connection with an inlet of said compressor and a condenser in fluid connection with an outlet of said compressor. The compressor is coupled to an operation unit capable of operating said compressor according to a method according to an aspect of the present invention, providing a compressor torque depending on a calculated compressor displacement.
Preferably, a temperature sensor is coupled to the evaporator. By this, an input temperature of the fluid being compressed in the compressor, particularly a refrigerant such as tetrafluorethane, can be measured. Favorably, a pressure on the high pressure side of the compressor can be measured with a pressure sensor which is coupled to said outlet of said compressor. The compressor is driven by a motor, such as an electric motor, or an engine, particularly a vehicle engine, and the engine speed can easily be measured with an appropriate speed sensor. With these signals, the displacement of the compressor can be calculated and a torque of the compressor estimated.
In a preferred embodiment, the compressor is in fluid connection with an automotive based air conditioning system.
According to another aspect of the invention, a computer program is proposed, comprising a computer program code adapted to perform a method for use in the preferred method according to any of the aspects described above, when said program is run on a programmable microcomputer.
The computer program can favorably be adapted to be downloaded to a control unit or one of its components when run on a computer which is connected to the internet.
Preferably, a computer program product stored on a computer readable medium, comprising a program code for use in a method according to anyone of the aspects of the preferred method on a computer.
According to another aspect of the invention, a computer program product stored on a computer readable medium is proposed, comprising a program code for use in a method comprising at least the steps of estimating a first thermodynamical state of said low pressure side and a second thermodynamical state of said high pressure side, estimating a current compression ratio of said compressor, calculating a compressor displacement as a function of said compression ratio, estimating a compressor torque depending on said calculated displacement, and providing said compressor torque to a control unit.